The present invention relates to a method for removing color from the effluent of a kraft pulp mill used to manufacture paper.
In the kraft pulping process, wood is cooked in a highly alkaline solution of sodium hydroxide and sodium sulfide. The cooking separates the cellulosic fibers contained in the wood from most of the lignin that binds the fibers together. The cellulosic fibers are separated from the cooking solution and then washed. The spent cooking solution, which is referred to as black liquor, is sent to a recovery device where the organic material in the solution is combusted and the inorganic chemicals are recovered for reuse.
The washed fibers, called brownstock pulp, have a light brown color from the lignin still bound to the fibers. If whiter pulp is desired for the paper product, the fiber must be bleached. Bleaching typically uses chlorine for the first stage of the bleaching reaction.
Chlorine is selective and reacts much faster with the lignin than the cellulose fibers. After chlorine bleaching, the fibers are separated from the spent chlorination process solution. The spent chlorination stage solution is strongly acidic with a pH of 1 to 2. This spent chlorination stage solution is also called chlorination stage filtrate, C stage filtrate, chlorination extract, or acid filtrate.
The chlorinated lignin compounds are no longer bound to the pulp, but they are not very soluble in water. Also, the lignins are somewhat sticky, so the lignins mostly stay with the pulp during the first filter stage. The second bleaching stage typically consists of adding a caustic solution (sodium hydroxide) to the pulp. This raises the pH and solubilizes the chlorinated lignin compounds. Because the lignin is now soluble, the chlorinated lignin can be extracted. The second stage is thus commonly called the extraction stage.
After the bleaching reaction is complete, the pulp is separated from the spent caustic solution. The spent caustic solution is weakly alkaline with a pH of 10 to 12. This spent caustic solution is also called caustic extract, E stage filtrate, or extraction stage filtrate. This filtrate is very highly colored because of chlorinated hydrocarbons (chromophores). In a modern paper mill, the caustic extract is up to 95% of the total color in the mill effluent, but only 5% of the total sewered flow.
For environmental reasons, it is desirable to remove the color from the mill effluent. This will improve the aesthetic appearance of the effluent discharged into a stream.